This invention relates to a cathode ray tube for use in projection color television, and more particularly relates to a method of forming a luminescent screen for such a tube.
Monochrome cathode ray tubes, for example, for projection television, employ a single electron gun mounted in the neck of the tube to focus a single electron beam on the luminescent display screen of the tube. A deflection yoke surrounding the neck of the tube, and associated electronic circuitry, cause the beam to scan the screen as well as to vary in intensity in response to a video signal to produce a monochrome display image.
In projection color television, three such displays, each in one of the primary colors, red, blue and green, are superimposed on a large projection screen to produce a full color display image. Because the images on the individual tube screens are not viewed directly, but are magnified and projected by a system of projection lenses, the individual cathode ray tubes are driven at higher loads than would be encountered for direct view tubes, in order to produce a full color display of acceptable brightness.
Projection tubes having interference filters designed to result in marked increases in luminous efficiency in the forward direction, as well as improved chromaticity and contrast, are described in U.S. Pat. Nos. 4,633,131, assigned to North American Philips Corporation, and 4,634,926, assigned to U.S. Philips Corporation, in which the filter is characterized as a short wave pass (SWP) filter and is composed of alternating layers of materials of high and low refractive index. Even further improvements are provided, especially in light gain in the corners of the display screen, by combining such an interference filter with an inwardly curved display window, as provided in U.S. Pat. No. 4,683,398, also assigned to U.S. Philips Corporation.
In the process of making these tubes, the interference filter is vapor deposited directly upon the inner curved surface of the faceplate. In order to provide maximum accessibility of this surface to the evaporation source, the face panel, with or without a peripheral sidewall or "skirt", and the funnel portions of the tube envelope are fabricated separately, and are sealed together after evaporation of the filter has been completed.
The luminescent phosphor screen is deposited directly on the interference filter. In theory, this could take place on the open panel before the panel and funnel are sealed together, such as by the slurry or dusting technique used in conventional direct view color tube manufacture, or by a settling technique similar to those used in direct view monochrome television tube manufacture.
However, whereas most direct view phosphor screens can have a few allowable defects in typical ranges from 10 mils to 40 mils in size, projection phosphor screens should be free of all defects in that range and preferably should be free of defects even in the range of 4 to 10 mils. The main reason for this "zero defect" requirement is that the projection display area is magnified, when projected, by a typical factor of about 100 to 1. Defects that look quite small to the eye on a 3".times.4" raster become quite large when magnified and projected onto a 30".times.40" or larger projection screen.
It is also true that uniform coating density or "screen weight", usually expressed in mg/cm.sup.2, is more important for projection tubes then for direct view monochrome. The light output of a given screen at each point is related to the coating density at that point. Since projection tube images are superimposed on one another, notable variations in screen weight from one point to another on the screens would result in poor white field uniformity.
It has been found in practice that screens having zero defects and uniform coating density cannot be produced repeatedly or reliabily on open panels by any of the above techniques. On the other hand, waiting to form the screen until after sealing of the panel to the funnel is not practical for these filter tubes, since access to the panel area for subsequent manufacturing operations needed to complete the tube is then limited to the open neck of the funnel.